1. Field of the Invention
This invention relates to an improved process for the gasification of coal. The process makes maximum use of run-of-the-mine coal and otherwise environmentally unattractive waste products for maximum recovery of useful gas products. In this era of energy shortage and need for protection of the environment, the need for an economical and effective process for coal gasification is vital.
2. Description of the Prior Art
The prior art can best be described by reference to descriptions of fixed bed non-slagging, air and oxygen blown gasifier systems on the one hand which utilize coarse coal as feed stock, and slagging, pressurized entrained flow gasifier systems on the other hand which utilize oxygen and a slurry of finely ground coal as charge stock, each of which systems constitute independent, self-contained units available for use according to the selection by a prospective operator.
Fixed bed gasification systems and entrained flow (or dilute phase) gasification systems are well-known. By the relatively fixed bed system, coal is converted into gas in a continuous process utilizing reasonably deep fixed beds, as exemplified in the conventional gas producer, or the Lurgi generator which operates with air-stream blasts or oxygen-steam blasts at pressures up to 30 to 50 or even higher atmospheres. Typical descriptions of fixed bed non-slagging gas generator systems are disclosed in Gas Engineers Handbook, 1974 by Industrial Press Inc., Chapter 9, pages 3/103 to 3/107, inclusive. Other references which discuss the Lurgi fixed bed type of gasifier are U.S. Pat. Nos. 2,834,665 to Rudolf, et al of May 13, 1958 and U.S. Pat. No. 2,799,565 to Just of July 16, 1957.
In the Lurgi system, coarse coal is intermittently fed to the gasifier through lockhoppers and gasification takes place in the downwardly moving bed of lump coal. This process has good thermal efficiency but has a major disadvantage in that the coal feed to the gasifier must be closely graded to eliminate coal fines which respresent a large proportion of the output from a coal mine, frequently on the order of 25 to 40 percent of the mine output. Other disadvantages are that the hydrocarbon by-products from the process require an inordinately large investment of capital and operating cost to refine them for selling into the market. Further, phenolic water and other dissolved organic materials discharged as waste from the process presents a severe disposal problem and equipment to extract it is both expensive and complex.
Gasification of coal by the entrained flow or dilute phase technique, utilizes a slurry of pulverized coal that is entrained in a gasification medium consisting of steam and/or water, plus oxygen and/or air. The mixture is blown continuously into a refractory-lined gasifier operating at atmospheric or higher pressures, a sequence of oxidation and steam decomposition reactions occurs. The product gas generated ranges in quality from producer gas to water gas or synthesis gas, depending upon oxygen concentration of the gasifying medium. The effective operation requires high temperatures, normally above the ash fusion point so that slagging conditions are readily obtained. This system has disadvantages because the coal feed must be completely pulverized and the oxygen requirement per unit product gas is very high.
Typical of the slagging pressurized entrained flow gasifier utilizing oxygen and a slurry of finely ground coal as charge stock are the following patents:
U.S. Pat. No. 2,761,824 to Eastman, et al Sept. 4, 1975 PA0 U.S. Pat. No. 2,595,234 to Eastman May 6, 1952 PA0 U.S. Pat. No. 3,715,195 to Tassoney, et al Feb. 6, 1973 PA0 U.S. Pat. No. 3,715,301 to Tassoney, et al Feb. 6, 1973
It is essential in each of the foregoing that finely ground or pulverized coal be used as feed stock after slurrying with a variety of liquids. Although several advantages are alleged, the process is not entirely satisfactory because it uses only pulverized coal and consumes large quantities of oxygen.
The only prior art known to applicant to a combination of two different gasification processes appears in the Gas Engineers Handbook 1974, Industrial Press Inc., Chapter 9, page 3/107, where brief reference is made to a new technique embodying a combined fixed bed and fluid bed operation that has been under development. However, the reference relates to the combination of two generators in parallel, both of which use various grades of finely divided fuel in a fluidized condition.
Other patents which show coal gasification processes wherein separate streams of coal are utilized are U.S. Pat. No. 4,017,270 to Funk, et al of Apr. 12, 1977 which refers to separate confined paths of coal particles utilized in a single gasifier, and U.S. Pat. Nos. 3,971,635 and 3,971,636, both to Matthews of July 27, 1976, which use a second stream of gas as fuel in the combustion zone and not as feed to a gasifier to create a source of gas for recovery of fuel.